Growth-Hormone Collagen Synthesis
GH doesn't build your skin directly. It bosses your liver into making IGF-1, and IGF-1 does the work.
Grounded in the research on Growth hormone (Wikipedia); Insulin-like growth factor 1 (Wikipedia)
What it actually is
Growth hormone (GH / somatotropin) is a 191-amino-acid peptide your anterior pituitary pulses out, mostly in deep sleep. Its job isn't to glue collagen together itself. It's an upstream signal. Collagen is the structural protein that makes up ~30% of your body's total protein and the bulk of skin, tendon, bone and cartilage. GH controls how fast you build it.
Stage 1 — the signal fires
Hypothalamus releases GHRH, pituitary dumps GH into the blood in pulses (biggest spike in slow-wave sleep, hence 'beauty sleep' is literal). GH is also driven up by exercise and deep sleep, and blunted by chronic high insulin/sugar. Miss sleep, flatten the pulse.